More precisely, it concerns a metal duct, the inside of which is subjected to steep temperature gradients, and which comprises a sheet metal tube of rectangular cross-section with rounded corners and reinforced with outer stiffeners constituted by members which are disposed in planes perpendicular to the axis of symmetry of the tube and which are connected thereto.
Conventionally, the tube is rectangular, or preferably square, in cross-section, with rounded corners so as to limit the stresses where they are at their maximum levels. The stiffening members which provide mechanical strength are constituted by members welded around the entire periphery of the sheet metal tube. Since the duct is subjected to steep temperature gradients, those faces of the stiffening members which are welded to the tube heat up very quickly while their outside portions are still at low temperatures. The differences in temperature on either side of the web of each of the members generate thermal stresses linked to the resulting differential expansion. At the corners, where the values of conventional mechanical stresses are at their maximum levels, the addition of the differential thermal stresses often causes the performance levels of the materials to be exceeded. As a result the stiffening members crack and more seriously the tube tears, and there is then a danger that leaks of the very hot medium being conveyed will occur, and that the damage will be made worse.
A known solution consists in lagging the outside of the stiffening members so as to limit the flow of heat through the duct sections and therefore the thermal differentials, but the inertia of the masses of the ducts considerably limits the performance levels of that solution.
Another known solution is to limit the height of the stiffening members so as to reduce the temperature difference between their bases and their tops, thereby requiring the stiffening pitch to be increased so as to retain the mechanical characteristics. This is not economically advantageous, and limits only the problem of fixing of the stiffening members, without guaranteeing the sealing strength of the ducts.